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Polyester resins are unsaturated resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Among other uses, it is the basic component of sheet moulding compound, bulk moulding compound and the toner of laser printers.

Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyhydric alcohols, organic compounds with multiple alcohol or hydroxy functional groups) and polycarboxylic that contain double bonds. Typical polyols used are glycols such as ethylene glycol. The usual polycarboxylic acids used are phthalic acid and maleic acid. Water, which is a by-product of this esterification reaction, is removed from the reaction mass as soon as it is formed to drive the reaction to completion.

Unsaturated polyesters differ from saturated polyesters such as polyethylene terephthalate, which constitutes the polyester films and fibers of commerce, in that acids or glycols having double bond unsaturation are included in the formula to provide reactive olefinic unsaturation in the unsaturated polyester. These are used to cross-link and harden the resin, in conjunction with a liquid monomer (nearly always styrene), which also lowers the viscosity of the resin to a useable extent, and an activating catalyst.

Polyester resins are thermosetting; "thermosetting" means the plastic softens when initially heated , but sets permanently rigid once it has cooled or has been chemically "cured/set"(as opposed to "thermoplastics", which re-soften with heat). Polyester resin is often purchased in liquid form for the production of glass-reinforced plastic. In this case, a catalyst (typically methyl ethyl ketone peroxide (MEKP) (also known as butanone peroxide) is used to initiate the polymerization reaction; benzoyl peroxide is a somewhat less hazardous alternative suitable for some purposes. The process of curing polyester resins using a catalyst is an exothermic process. The use of excessive catalyst can cause an excess of heat during the cure process and thus damage the resin by charring or even cause ignition.

Contents 1 Uses of Polyesters 2 Health and Environmental Issues 3 Types of Unsaturated Polyester Resins 4 See also 5 References

[edit] Uses of Polyesters

Polyester resin is used for boat hulls and car bodies etc. (with glass fibre or carbon fibre), auto body repair, wood filling, casting, and as an adhesive. It has good wear and adhesive properties, and can be used to repair and bond together many different types of materials. Polyester resin has good longevity, fair UV resistance, and good resistance to water. It is important to recognize that all polyester resin products are not created equal; their chemical makeup is complex and they can have a wide range of properties, many depending on additives such as fillers, reinforcements, etc. As a filler in auto repair, for example, this material is formulated for superior adhesion to paints and metals, but cures very hard to resist surface trauma; it is therefore only marginally sandable. As a filler for millwork, however, polyester must be softer than the wood substrate so it can be sanded without leaving fills standing proud above surfaces.

Polyester resin's adhesion to some materials can be excellent such as in fiberglass composites but others such as timber the adhesion is much weaker.

When polyester is used as a laminating medium, glass or carbon fibre is often added for strength. Fiberglass can either be chopped up to give the final product internal strength, or laid-up in polyester-saturated strips to create high-strength external forms. A viscous polyester "gelcoat" is first brushed or sprayed into the mold to create a smooth, coloured outer surface. The fiberglass-reinforced polyester is applied into this surface coat to give the strength, volume, and rigidity. Laminating occurs as follows:

1. A mold is made for the desired shape
2. Mold-release is applied to keep the casting from adhering to the mold
3. The pre-catalysed gelcoat is brushed or sprayed into the mold to create the final surface
4. A catalyst such as butanone peroxide is added to the liquid polyester resin to promote polymerisation
5. The reinforcing fibre in the form of a mat or sprayed chopped strands is applied on the cured gelcoat and impregnated with a roller
6. After resin has cured, the laminate is pulled from the mold.

This molding process is used in a variety of applications including: Boat hulls, car and truck bodies, components for film set design, auto prototyping, and the creation of lost architectural details such as plaster rosettes, etc.

[edit] Health and Environmental Issues

The drive to be environment-conscious has led also to the development of water-extendable polyester resins and LSE (Low Styrene Emission) polyester resins. ^[1]

Partially replacing styrene monomer, water reduces the resin solids and adjust working viscosity. This reduction of styrene fume and odor greatly benefits the workers' health and safety.

LSE resins allow the workers to improve productivity without the unnecessary exposure to styrene emissions.

[edit] Types of Unsaturated Polyester Resins

chemical resistant resins clear and filled casting type resins cold press type resins continuous laminating resins filament winding resins flame retardant resins general purpose resins hand lay-up resins hot press bmc resins hot press smc resins low styrene emission (LSE) resins| plasticizing flexible resins pultrusion type resins putty resins resins for electric motors resins for mold production rtm light type resins rtm type resins sewer rehabilitation-renewing resins solid surface resins spray-up resins translucent resins vacflo resins vacuum bagging resins vacuum injection resins.[2]

[edit] See also

• Polyresin

[edit] References

1. ^ "Water-Extendable Resins". http://chemrez.com/default.asp?link=FiberComposite. 
2. ^ "Polipol Polyester Resins". Poliya. 2007. http://www.poliya.com.tr/poliya_products_polipol_polyester_resin.htm. Retrieved 2007-05-08.